1. Field of the Invention
The present invention relates to a Semiconductor Bulk Acoustic Resonator (SBAR) or Bulk Acoustic Wave (BAW) resonator configured to reduce lateral acoustic modes in order to improve the insertion loss and phase characteristics of SBAR filters.
2. Description of the Prior Art
Semiconductor bulk acoustic resonators (SBARs) are known in the art. An example of such an SBAR is disclosed in commonly owned U.S. Pat. No. 5,382,930; incorporated by reference. SBAR devices include resonators, stacked crystal filters (SCFs), multipole filters based on inductor coupled topologies disclosed in the '930 patent and other filter topologies. Because of their relatively small size, high Q, and high operating frequency, such SBARs are particularly well suited in high performance applications where space is limited.
Such SBARs are thin film acoustic resonators fabricated on a semiconductor substrate, such as gallium arsenide (GaAs), making them particularly suitable for integration with high electron mobility transistor (HEMT) and heterojunction bipolar transistor (HBT) microwave monolithic integrated circuits (MMICs), for example, as disclosed in: "SBAR Filter Monolithically Integrated With an HBT Amplifier", Cushman et al., IEEE Ultrasonic Symposium, 1990, pp. 519-524; and "SBAR-HEMT Monolithic Receiver Front End With Bulk Acoustic Filters", Cushman et al., GOMAC Digest, 1997, pp. 279-282; hereby incorporated by reference. Such SBARs typically include a thin layer or film of a piezoelectric material, such as aluminum nitride or zinc oxide which may be deposited on the semiconductor substrate, for example, by sputtering. Thin film metal electrodes are formed on opposite surfaces of the piezoelectric layer to form an SBAR resonator. A stacked crystal filter (SCF) is similar but includes two AIN layers and three metal electrodes.
Irregularities of insertion loss and phase have heretofore prevented such SBARs from being used in many applications. Such performance irregularities are due to the undesired lateral acoustic modes of such SBARs. These lateral acoustic modes are known to interfere with the desired longitudinal acoustic mode thereby causing the performance irregularities discussed above.